There are four major types of mask image features. They are line/space arrays, isolated lines, isolated spaces, and contact holes. A common problem encountered in lithography methods is that prior art resolution or DOF image-enhancement techniques do not uniformly enhance all four major types. Instead, prior art techniques provide best enhancement for specific major feature types so that some features are enhanced relative to others in the same design pattern. These fully enhanced feature types are referred to hereinafter as enhanced. Features not fully enhanced by a particular technique are referred to hereinafter as unenhanced. Because prior art techniques favor selectively enhanced particular features types, uniform mask image enhancement for mixed design patterns, i.e., with many feature types, is very difficult.
The well-known off-axis illumination method, for example, enhances closely spaced pattern images such as line/space arrays much more than isolated features or contact holes. By contrast, the attenuated Phase Shift Mask (PSM) method, results in better resolution and DOF for isolated features and contact holes than for line/space arrays. The Rim PSM method is similar to the attenuated PSM method in the feature types enhanced. Prior art image enhancement techniques generally provide only selective enhancement of certain feature types which do not result in uniform image enhancement, or pose considerable design and/or manufacturability difficulties.